This invention relates to a slit-scanning type optical copying apparatus, in which successive increments of the surface of an original are scanned by reciprocating an original carrying member such as an original support, an original carrier sheet and the like, or an original-image scanning mirror, so that an original image is focused on a photosensitive member moving or rotating at a given speed or at a given R.P.M. in a given direction, through a projecting optical system including an objective lens.
The prior art slit-scanning type optical copying apparatus poses a problem that because of a given direction of movement or rotation of a photosensitive member, a desired correct image may not be obtained during one of the reciprocal travels of the original-carrying support or the original-image scanning mirror.
As a result, the presently known copying apparatus of this type fails to produce copies during both the advancing and returning travels of the support or mirror, so that reproduction is effected only during one of the aforesaid advancing and returning travels, for instance, during the advancing travel of an original carrying member or an original-image scanning mirror. Thus, the other travel in the direction opposite thereto serves only as a return travel of the support or mirror in preparation for the next reproduction, thus effecting a lowered copying efficiency.
U.S. Pat. No. 3,574,459 and German Offenlegungsschrift No. 2,358,370 propose mechanisms for avoiding the foregoing shortcomings by providing reproduction which may be effected during both reciprocal travels of the original support. U.S. Pat. No. 3,574,459 uses a projecting optical system including a pair of objective lenses and a single Abbe prism positioned between the lenses, and the Abbe prism is turned through an angle of 90.degree. in response to the advancing and returning travels of the support, thus enabling reproductions during both reciprocal travels of the support.
On the other hand, German Offenlegungsschrift No. 2,358,370 uses a projecting optical system including a single objective lens and two mirrors or prisms of a triangular cross section, which are positioned in the rear of the aforesaid lens and have different reflecting angles, with the direction of one of the mirrors or prisms being perpendicular to that of the other, whereby the two mirrors or prisms alternately intercept the optical path of the lens in response to reciprocal travels of the support, thus achieving reproductions during both reciprocal travels of the support.
With the former proposed mechanism, a reflecting member is used for dual purposes of reciprocal movements and reflection and thus is superior to the latter in that the reflecting member alone is simply turned through an angle of 90.degree.. However, this suffers from a shortcoming in that the volume of the reflecting member or Abbe prism is relatively bulky and two objective lenses have to be used, thus resulting in complex construction and failure in providing a compact arrangement.
On the other hand, the latter proposed mechanism is superior to the former in that the reflecting members, i.e., mirrors or prisms having triangular cross sections are of a smaller volume, but pose the problem that two reflecting members must be selectively used.
In short, the prior art attempts necessarily use two objective lenses or two reflecting members having triangular cross sections, thus resulting in complexity of the projecting optical system and an increase in the space required therefor, coupled with an increase in manufacturing cost, as compared with presently available copying apparatuses.